The production of solid polymer particles which are suitable for pelletization and use in subsequent manufacturing processes may be accomplished by any one of many known polymerization techniques in a polymerization reactor. In such processes, the polymer slurry effluent from the reactor typically consists of the particulate polymer formed in the reactor suspended in liquid hydrocarbon diluent, which acts as the polymerization reaction medium. The diluent may be an inert solvent or excess monomer or comonomer. As an example, when ethylene is polymerized in a hydrocarbon diluent such as isobutane under controlled temperature and pressure conditions, a slurry of polymer solids and diluent is formed. This type of process is known as particle form polymerization. One drawback of this kind of process, or any process in which a polymer is prepared in solution and subsequently precipitated to form a slurry, is that the solid polymer must be separated from the liquid portion of the slurry. This liquid portion may include any suitable solvent(s) (diluent) utilized in the particular polymerization process (typically these are C.sub.3 -C.sub.8 hydrocarbons) and/or unpolymerized monomer or comonomer, all of which will be hereinafter referred to collectively as "hydrocarbon." The term "hydrocarbon" is not intended to include the polymer itself.
In certain polymer recovery processes, which are known as "wet" processes, water is added to the polymer slurry as it exits the reactor to facilitate pumping the slurry through the polymer recovery system. Improvements in hydrocarbon removal from polymer slurries in "wet" recovery processes are the subject of copending application Ser. No. 07/739,876, filed Aug. 2, 1991, now U.S. Pat. No. 5,207,929, issued May 4, 1993, naming Chieh-Yuan F. Sung and Stephen Krause as coinventors. The methods and apparatus disclosed in the referenced copending application provide excellent results in terms of reduction of hydrocarbon emissions. Such methods and apparatus may require additional energy expenditures, however, since the water initially added to the slurry must subsequently be removed.
In other conventional polymer recovery processes, which are known as "dry" processes, no water is added to the polymer slurry. In an example of a conventional dry process, polymer slurry from a polymerization reactor is fed to a flash tank operated at low pressure (on the order of 1-2 psig) to flash a portion of the hydrocarbon therefrom. Recovery of the flashed hydrocarbon vapor typically requires recompression prior to recovery. The recompression of the vapor is an energy intensive step which adds significantly to the energy consumption and cost of the overall process. Subsequently, polymer powder from the flash tank is fed to a heated mechanical conveyor dryer which vaporizes additional hydrocarbon from the polymer powder. Although this drying step removes some additional hydrocarbon from the polymer powder, a significant amount of hydrocarbon remains in the powder. The polymer is then conveyed to a powder silo via a purge conveyor which utilizes nitrogen as a purge gas to remove additional hydrocarbon. Because the polymer output from the previous drying step contains substantial unvaporized hydrocarbon, the nitrogen purge gas in the purge conveyor picks up a substantial amount of hydrocarbon which must subsequently be removed from the purge gas. This is accomplished by compression and condensation, which further add to the overall energy consumption and costs of the recovery process.
What is needed is a polymer recovery process which is both highly energy efficient and serves to significantly reduce hydrocarbon emissions to the atmosphere.